Musculoskeletal disorders including fibromyalgia, temporomandibular disorders and myofasical pain syndrome are significant health problems interfering with normal occupational and social activities. The long term goal of this research is to understand the neuroplastic changes that occur in these musculoskeletal disorders and to provide insight into potential diagnostic and therapeutic strategies based upon knowledge gained from studies of experimental animal models. The hypothesis is that acidification of the extracellular environment within muscle alters the cellular and molecular properties of muscle primary afferent and second order neurons resulting in hyperalgesia. It is expected that muscle acidity will induce an increase in number of peptidergic neurons via a phenotypic switch within the primary afferent population with the non-spindle group II and those group III, IV muscle afferent neurons that normally do not express peptides, now expressing substance P (SP) and calcitonin gene-related peptide (CGRP) and contributing to muscle hyperalgesia. Aim 1&2: To determine the number and size of muscle afferent neurons in the trigeminal ganglion (TG) and leg muscle afferent neurons in dorsal root ganglion (DRG) which express SP, CGRP and the number of neurons expressing receptors for SP (NK-1) and CGRP in the medullary and lumbar spinal dorsal horn in experimental and control animals. This aim will be accomplished by combined retrograde neuronal labeling, immunohistochemistry and image analysis to quantitatively analyze peptidergic muscle afferent somata within the TG (following masseter muscle injection) and lumbar DRG (gastrocnemius muscle injection) and receptor expressing neurons in the medullary and lumbar spinal dorsal horn. Aim3: To compare gene expression for CGRP, SP within the TG and lumbar DRG using reverse transcriptase polymerase chain reaction (RT-PCR). Chemical interventions (CGRP, NK-1 antagonists) will also be introduced during chronic muscle hyperalgesia. Agents capable of modulating SP, CGRP expression following decreased tissue pH may have potential not only as acute therapeutics but potentially as modulators of longer term nociceptive phenomena. [unreadable] [unreadable]